import java.util.Scanner;

public class demo01 {
    //不要二
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        int m=sc.nextInt();
        int count=0;
        int[][]arr=new int[n][m];
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                if(arr[i][j]==0){
                    arr[i][j]=1;
                    count++;
                    if(i+2<n){
                        arr[i+2][j]=1;
                    } else if (j+2<m) {
                        arr[i][j+2]=1;
                    }
                }
            }
        }
        System.out.println(count);
    }
}
